1. Field of the Invention
The present invention relates to a guide vane assembly for a gas turbine engine. In particular, aspects of the invention relate to a composite guide vane assembly and arrangements for attaching composite guide vane assemblies to a gas turbine engine.
2. Description of the Related Art
An axially rotating gas turbine engine comprises a number of rotor stages and a number of stator stages, each comprising aerofoil members mounted around an engine axis. The rotor stages are arranged such that, in use, the aerofoil members rotate around the engine axis. The stator stages are arranged such that, in use, the aerofoil members are fixed in position around the engine axis. Conventionally, a gas turbine engine would have a compressor and a turbine, each of which would comprise at least one (and usually several) rotor and stator stages.
A cut-away diagram of an example of a turbofan (bypass) gas turbine engine 50 is shown in FIG. 1. The gas turbine engine 50 comprises, in axial flow series, a fan 12, a compressor 60, a combustor 15, and a turbine 70, the operation of each of which is well known. The compressor 60 and turbine 70 are conventionally referred to as being part of the core of the engine, and the flow from the fan that does not pass through the core of the engine is said to pass through a bypass duct 22.
The compressor 60 and turbine 70 both have rotor stages and stator stages. The fan 12 may be considered as a rotor stage. In the FIG. 1 example, a stator stage 32 is provided in the bypass duct 22 for the flow that does not pass through the core of the engine. This stator stage in the bypass duct 22 may be referred to as an outlet (or outer) guide vane (OGV) stage 32.
A more detailed view of an OGV stage 32 is shown in FIG. 2. Conventionally, OGVs are manufactured using a metal, including the aerofoil members (or vanes). The OGVs are conventionally manufactured as individual vanes, and an example of a conventional OGV 34 is shown in FIG. 3. Conventionally, the individual OGVs are mechanically fixed at their ends to an inner ring 37 and an outer ring 38 using a mechanical fastener 36, such as a bolt, in order to form the OGV stage 32 shown in FIG. 2. For example, the OGV 34 may be provided with a platform 35, at one or both of its ends, and the platform(s) may then be bolted to the inner and/or outer rings 37, 38. In the FIG. 3 example, a platform 35 is provided at an inner radius to bolt the OGV 34 to form the inner ring 37, but the OGV 34 is directly bolted to the outer ring 38 at the outer radius.
However, metallic OGVs 34 and the conventional arrangements for assembling metallic OGVs 34 to form an OGV stage 32 are very heavy.
It is desirable to replace the conventional metallic OGVs 34 with lighter components. In turn, this would result in a lighter OGV stage, and thus a lighter engine, and improved fuel consumption (or SFC).
It is proposed to use a composite structure rather than a metallic structure to form the OGVs. Such a composite structure may have sufficient strength, but considerably lower weight than a conventional metallic OGV. Composite OGVs may also be less expensive to manufacture than metallic OGVs.
However, conventional approaches to manufacturing/assembling an OGV stage may not be appropriate when using composite OGVs. For example, as mentioned above in relation to FIGS. 2 and 3, the conventional metallic OGVs 34 may have platforms 35 formed at either or both ends, and may be bolted to the inner and outer rings 37, 38 individually. Composite materials may not be well suited to such fixing arrangements. For example, bolting straight through composite material may be expensive/time-consuming and/or may significantly weaken the component. This may compromise the design and/or result in shorter life spans.
For example, in a flange-type fixing, bolting through flanges can introduce potential weak points in any resin rich areas and may add manufacturing complexity. Furthermore, drilling fixing holes through any loaded area of a composite structure is not desirable as it may introduce discontinuities to some of the fibres. Using a dovetail-type fixing may require a rapid build-up of thickness of the composite material, making the manufacturing process more complex and potentially introducing weakness at ply drop interfaces.
Although much of the discussion above and elsewhere herein focuses on OGVs, it will be appreciated that the same or similar considerations may apply to other stator stages and guide vanes therefor.